Dynamics and Shocks from H$\alpha$ Emission of Nearby Galaxy Mergers

TL;DR

This study investigates the dynamical behavior and shock properties in 22 nearby galaxy mergers using integral field spectroscopy, revealing how shock fractions vary with interaction stage and mass ratio, and providing new insights into merger dynamics.

Contribution

It introduces a novel method to estimate shock fractions and encounter parameters in galaxy mergers using ext{H} ext{α} velocity maps and kinematic modeling, including the first constraints on encounter parameters.

Findings

Higher shock fractions in close galaxy pairs compared to wide pairs.

Coalesced mergers exhibit the highest average shock fraction.

More equal mass ratio pairs tend to have higher shock fractions.

Abstract

We examine the dynamical properties of interacting galaxies and the properties of shocked gas produced as a result of the interaction. We observed 22 galaxy mergers using the SparsePak IFU at Kitt Peak National Observatory (KPNO). The goal of the observations was to obtain the \ha\ velocity maps over the entire luminous parts of the galaxies including the faint tidal tails and to find extended shocks and outflows. Our sample consists of major and minor galaxy mergers with mass ratios $1<\mu<8$. We fit multiple kinematic components to the \ha\ and \nii\ emission lines, develop an MCMC code to robustly estimate the error of fit parameters, and use the F-test to determine the best number of kinematic components for each fiber. We use \nii/\ha\ and velocity dispersion of components to separate star-forming (HII) regions from shocks. We use the kinematics of the \ha\ emission from HII regions and an automated modeling method to put the first-ever constraints on the encounter parameters of one of the observed systems. Besides, we roughly estimate the fraction of shocked \ha\ emission , $\text{f}_\text{shocked}$, without taking extinction into account and examine the spatial distribution of shocks. We find that close galaxy pairs have, on average, a higher shock fraction than wide pairs, and coalesced mergers have the highest average $\text{f}_\text{shocked}$. In addition, galaxy pairs with more equal mass ratio tend to have a higher $\text{f}_\text{shocked}$. Combining the dynamical models from the literature and this work, we inspect trends between $\text{f}_\text{shocked}$ and dynamical encounter parameters. Our findings are generally consistent with shocks being produced either by the direct collision of the ISM or by the chain of events provoked by the tidal impulse during the first passage.